RF coupling impedance measurements for particle accelerator devices

Bench measurements nowadays represent an important tool to estimate the coupling impedance of any particle accelerator device. The well-known technique based on the coaxial wire method allows to excite in the device under test a field similar to the one generated by an ultra-relativistic point charge. We discuss the basics of the coaxial wire method and review the formulae widely used to convert measured scattering parameters to longitudinal and transverse impedance data. We review, as well, bead-pull technique used in the design, construction and tuning of multi-cell accelerating structures. We discuss typical measurement examples of interest for the CERN Large Hadron Collider as well as other state of the art particle accelerator.